Non-destructive testing of large diameter steel pipes such as natural gas pipelines using magnetic-inspection techniques is well known in the art. Tools fitted with magnetic flux leakage (MFL) anomaly detectors embodying strong permanent magnets to magnetize the pipe wall to near saturation flux density are generally employed. Sensors, moving with the detectors, have signals induced by variations in the leakage fields caused by such pipe-wall anomalies as internal or external corrosion, hard spots and so on, and including local variations in the magnetic permeability. MFL anomaly detectors are, however, subject to problems caused by such things as noise, hostile operating environment, restricted power due to battery capacity and finite data storage capacity. Thick walled or small diameter pipes are more difficult to inspect by the MFL anomaly technique because there is frequently little space available inside such pipes for magnets compared to the relatively large pipe-wall cross sectional area to be driven into magnetic saturation. While this problem has been alleviated in recent years following the introduction of neodymium-iron-boron (NdFeB) permanent magnets which have improved mechanical and magnetic properties and can produce higher usable flux densities than the earlier ferrite magnets and are mechanically stronger than cobalt-rare earth magnets, MFL anomaly techniques are generally limited to essentially direct contact inspection and are not generally suitable for indirect coupling through air or other materials or when there is appreciable sensor liftoff. Remote field eddy-current (RFEC) devices have been developed for these latter tasks. RFEC devices incorporate, transmit and receive coils,and use a solenoidal exciter coil, energized with low frequency ac (typically, 20-2000 Hertz), internal to and generally approximately coaxial with the longitudinal axis of the pipe to be tested. It has been found that the operating frequencies for non-ferromagnetic tubes, such as reactor pressure tubes, are much higher at about 10 kHz, than those suitable for ferromagnetic pipes which are typically about 60 Hz. Low frequencies imply low scanning speeds. The detector coil, or array of detector coils, is placed adjacent the inside of the pipe wall and axially or radially aligned. The exciter coil or coils are displaced longitudinally along the pipe from the exciter coil by about 2 to 3 pipe diameters (D). At this separation direct coupling between the exciter and the detector is strongly attenuated by the pipe. The signal in the detector results principally from an indirect energy transmission path on the outside of the pipe. Field from the exciter diffuses through the pipe wall in the vicinity of the exciter, being attenuated and phase shifted in the process. Once on the outside, this energy then radiates with relatively little attenuation. In the case of a ferromagnetic pipe, it tends to be guided preferentially in the axial direction.
Adjacent the remote detector coil, the field on the outside of the pipe is much greater than the field inside, which is generated largely by energy which diffuses back from the outside, again being attenuated and further phase shifted in the process. Anomalies anywhere in this energy-transmission path will cause changes in the phase and amplitude of the received signal. Because the received signals are small, typically of the order of 10 .mu.v, a phase sensitive synchronous detector or lock-in amplifier is incorporated to receive and amplify the signal.
While RFEC probes have now been used for many years for well-casing inspection and more recently for heat exchangers and pressure tubes, the phenomenon is complex and defect responses are still not fully understood.
Large diameter concrete lined steel pipes or cylinders have been used to convey water for many years and such pipes are frequently provided with a spirally wound high strength prestressing wire which is pre-tensioned before a top coating of mortar is applied. In some instances a layer of concrete is also applied to the outside of the steel pipe before the prestressing rod, bar or wire is applied and a second layer of mortar is applied on top of the rod, bar or wire spiral. While rupture of prestressed concrete pipe is relatively uncommon, nevertheless periodic inspection of municipal water supply pipes and the like, which have an expected service life of 30 years or more, would be advantageous in order to prevent expensive ruptures or other failures. Heretofore, however, there has not been any practical method for inspecting composite pipes, such as prestressed concrete pipes.